The flexible expanded graphite sheet itself has been traditionally well known. This graphite sheet is usually manufactured in the following manner. Natural flake graphite, kish graphite, or the like is subjected to an oxidation treatment by anodic oxidation or immersion in an acid (for example, a mixed acid in which nitric acid or the like is added to a concentrated sulfuric acid), and the resultant material is washed with water and then dried, followed by a heating and expanding process to produce expanded graphite. The expanded graphite is compression-molded by a press machine or a rolling machine. This expanded graphite sheet shows not only excellent chemical resistance, heat resistance, heat and electrical conductivity, which are the features of graphite, but also high flexibility and compression recovery, and moreover has large anisotropy. Therefore, the expanded graphite sheet is widely used for various packing materials, high-temperature heat insulating materials, and heat radiating materials.
However, the just-described expanded graphite sheet contains impurities such as Si as well as Fe and Al in large amounts because the starting material thereof is natural flake graphite or kish graphite. Moreover, because it is produced through the immersion treatment in the concentrated-sulfuric-acid-based mixed acid, it has a significant drawback that sulfur compounds remain therein in large amounts and the content of S is particularly large. For this reason, there is a drawback that, when this expanded graphite sheet is used under such conditions as a heated condition, reduced pressure, or gas substitution, the atmosphere is contaminated because of these impurities. This drawback tends to be particularly evident especially when the content of S as an impurity is high.
In view of the foregoing, the present inventors have proposed a flexible, high-purity expanded graphite sheet in which the content of S is 15 ppm or less and the total impurity content other than S is 20 ppm or less.